Multilevel interconnects in semiconductor integrated circuits are formed principally from Cu interconnect lines and insulating layers. The Cu interconnect lines embedded in the insulating layers are covered on their sides and bottoms by barrier layers of Ta/TaN or the like, i.e., Cu diffusion prevention films, and their top surfaces are covered by cap layers of SiN, SiCN, or the like. The purpose of forming such cap layers is to prevent oxidation of the Cu interconnect surfaces when forming an insulating layer thereon and to prevent Cu diffusion between adjacent Cu interconnects after forming the insulating layer.
However, since the adhesion between the Cu interconnect surface and the cap layer formed thereon is poor, and the interface may thus serve as a diffusion path for Cu atoms, causing electromigration failure or time-dependent dielectric breakdown (TDDB), the present state of the art of cap layer material has a problem from the standpoint of reliability. It is therefore necessary to provide a cap layer material having good adhesion and barrier characteristics, as well as good oxidation resistance and a process for fabricating the same.
In the present state of the art, the cap layer is formed over the entire surface of each layer, i.e., not only on the Cu interconnects but also on the insulating layer. The dielectric constant of such cap layers is 3 to 7, and because of the presence of the cap layer over the entire layer interface, the dielectric constant of the insulating layer tends to increase, contrary to the requirement that the dielectric constant be reduced; therefore, forming the cap layer over the entire surface is not desirable in view of interconnect delays in logic circuits or memory circuits. Accordingly, it is desirable to provide a cap layer material that can exhibit the desired characteristics when the cap layer is formed only on the interconnect surface, not over the entire layer interface, and a process for fabricating the same.
The presence of the cap layer over the entire layer interface is also not desirable when applications to CMOS image sensors are considered. In image sensor applications, the cap layer which, in the present state of the art, is formed over the entire layer interface has the problem that the amount of light that reaches the sensor area decreases because of the high visible light reflectivity of the insulating layer. Accordingly, it is desirable to provide a cap layer material such that the cap layer can be formed only on the interconnect surface without leaving any highly reflective material in the insulating layer, and a process for fabricating the same.
Various attempts have been made to solve the above problems. For example, Non-patent Document 1 reports that when the cap layer is formed by immersion-plating the Cu surface with CoWP, the adhesion of the interface improves, and the electromigration lifetime increases. However, since this process usually requires that the plating be applied at temperatures of 70° C. or higher, it is extremely difficult to manage the plating solution, and the reproducibility of the manufacturing process is difficult to achieve. Furthermore, since CoWP does not have oxidation resistance, CoWP only serves to improve the adhesion, and a conventional cap layer of SiCN or the like has to be formed over the entire interface in an additional process in order to provide the necessary oxidation resistance. Further, if impurities exist on the surface of the insulating layer, CoWP is also deposited on them, causing problems such as leakage between interconnects and degradation of TDDB characteristics, and these problems become more serious as the pattern feature size decreases by reducing the spacing between interconnects.
Non-patent Document 2 reports that when a CuSiN layer is formed on the Cu surface by the reaction of SiH4 and NH3, the EM lifetime improves. However, when forming a Cu—Si solid solution on the Cu surface by using SiH4, if the Si content is too high, Si residues will remain in Cu even after nitriding Si with NH3 in the subsequent step, and this can cause an increase in resistance. Furthermore, since CuSiN itself does not have oxidation resistance, a conventional cap layer of SiCN or the like has to be formed over the entire interface in order to provide the necessary oxidation resistance.
Patent Document 1 proposes a technique that improves the reliability of interconnects by using a metal such as Sb, In, Sn, Hf, Ti, or the like for forming a cap layer in a multilevel interconnect structure comprising interconnects formed of Cu or a Cu alloy and insulating layers formed of dielectric material. According to the technique proposed in this document, the metal for forming the cap layer is deposited on the exposed surfaces of the Cu interconnect and the insulating layer, and the cap layer metal on the surface of the insulating layer is oxidized when deposited thereon, while the cap layer metal deposited on the surface of the Cu interconnect is diffused into the Cu interconnect to form a solid solution or an intermetallic compound. It is claimed that the solid-solutioned cap layer metal is segregated along grain boundaries in the Cu interconnect or at the interface between the Cu interconnect and the upper dielectric material, slowing the diffusion rate of Cu and thus serving to reduce electromigration or stress migration related failures.
It is also claimed that metals such as Al and Ti when deposited on the Cu interconnect surface form oxides by just being exposed to air, and that these oxides cannot improve the reliability of the interconnects. Accordingly, it is claimed that rather than forming oxide on the Cu interconnect surface, it is preferable to allow the cap layer metal to form a solid solution. Furthermore, since the resistance of the Cu interconnect increases when the cap layer metal is solid-solutioned into pure Cu, the thickness of the metal cap layer is held within a range of 0.5 nm to 5.0 nm, thereby holding the solid solution at a low concentration to suppress the increase of the resistance.
However, such a low-concentration solid solution does not provide the necessary oxidation resistance when the interconnect is exposed to an oxidizing atmosphere, and therefore has the shortcoming that the Cu interconnect is internally oxidized during the process of forming the upper insulating layer on the Cu interconnect. There is the further problem that after the insulating layer is formed, diffusion occurs between the Cu interconnect and the upper insulating layer in a subsequent high-temperature process. Furthermore, with the oxide of the cap metal formed on the surface of the insulating layer, it is difficult to obtain good characteristics as an insulating film, and it is therefore extremely difficult to ensure good interconnect leakage current and TDDB characteristics; accordingly, the proposed technique is not a realistic one. Further, since the layers formed here are high dielectric constant layers, the dielectric constant of the insulating layer as a whole increases, resulting in the problem that the interconnect propagation delay increases.
Though not directly related to the above-described semiconductor interconnects, it is generally known that the internal oxidation of Cu can be suppressed by coating the Cu surface with Sn, but no such known techniques are concerned with oxidizing the Sn-plated layer for use. For example, in Patent Document 2, a material having excellent oxidation resistance is fabricated by plating the inside of a copper pipe with Sn. Since preferential oxidation occurs when pin holes are formed in the Sn plating, a 1-μm thick Sn-plated layer free from pin holes is formed by controlling the concentration of each component in the plating solution.
On the other hand, in Patent Document 3, a layer of Ni or Cu or an alloy thereof is plated as an intermediate layer onto the surface of a copper base, and Sn plating is applied on top of that, thereby forming a plated layer with an Sn—Cu intermetallic compound dispersed therein. This uppermost layer is formed to a thickness of 0.5 μm or greater to provide the necessary oxidation resistance. Further, in Patent Document 4, an intermetallic compound diffusion layer containing Cu and Sn is formed to a thickness smaller than 0.2 μm by Sn immersion plating and, on top of that, a layer of benzotriazole or a derivative thereof is formed as a corrosion inhibitor layer to provide the necessary oxidation resistance.
In this way, in the prior art Sn plating on a bulk material, pure Sn or an Sn—Cu intermetallic compound is formed, but in order to provide the necessary oxidation resistance, the layer thickness has to be reduced to a micron-order thickness, and when the thickness is reduced, an intermediate layer or a surface layer has to be additionally formed.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2006-203197    [Patent Document 2] Japanese Unexamined Patent Publication No. H10-18045    [Patent Document 3] Japanese Unexamined Patent Publication No. 2003-82499    [Patent Document 4] Japanese Unexamined Patent Publication No. 2006-319269    [Non-patent Document 1] C. K. Hu et al., Microelec. Eng., 70, 406 (2003)    [Non-patent Document 2] S. Chhun et al., Microelec. Eng., 76, 106 (2004)    [Non-patent Document 3] F. DeCarli and N. Collari, Metallurg. ital. 44 (1952) 178
As described above, in the fabrication of Cu multilevel interconnects for semiconductor integrated circuits, attempts have been made to form a cap layer only at the interface between the Cu interconnect and the upper insulating layer by forming a CoWP layer or a CuSiN layer, but these layers have neither oxidation resistance nor barrier properties, and the use of such layers only serves to improve the adhesion of the interface between the conventional SiN, SiCN, or SiC cap layer and the underlying Cu interconnect; accordingly, the conventional cap layer has to be formed over the entire layer interface. This has been a major obstacle to reducing the dielectric constant of the insulating layer. Further, the method has been proposed that diffuses a metal, such as Sb, In, Sn, Hf, Ti, or the like, into the Cu interconnect to form a solid solution or an intermetallic compound therein, while on the other hand, forming oxides of such metals on the surface of the insulating layer, but this method not only has the problem that the Cu interconnect resistance increases, but also has the problems that the oxidation of the Cu surface cannot be prevented and that the dielectric constant of the insulating layer increases.
In view of the above enumerated problems, a first challenge of the present invention is to form a novel interface layer at the interface between the insulating layer and the underlying Cu interconnect and to confer excellent adhesion, oxidation resistance, and diffusion barrier characteristics to this novel layer. A second challenge is to hold the dielectric constant of the insulating layer as a whole to a low value compared with the prior art structure by forming the novel interface layer only on the Cu interconnect. A third challenge is to prevent the resistance of the Cu interconnect from significantly increasing after forming the novel interface layer, compared with that of the prior art Cu interconnect. It is an object of the present invention to provide a novel interface layer and a process for fabricating the same that address the above challenges.
To address the above challenges, there is provided a semiconductor device having a multilevel interconnect structure comprising: a first insulating layer formed on a semiconductor wafer; a Cu interconnect layer formed on a surface of the first insulating layer; a second insulating layer formed on the Cu interconnect layer; and a metal oxide layer formed at an interface between the Cu interconnect layer and the second insulating layer.
In the above structure, the metal oxide layer may be an Sn oxide or Zn oxide layer. Further, the metal oxide layer may be formed by first depositing a metal layer selectively on the Cu interconnect layer by immersion plating, and then heat-treating the metal layer in an oxygen-containing atmosphere.
In the above structure, the metal oxide layer may be formed to a thickness not smaller than 5 nm but not greater than 50 nm. Further, the Cu interconnect layer may be formed so that the concentration of metal atoms forming the metal oxide layer and diffused into the Cu interconnect layer to form a solid solution therein does not exceed 2% by atomic fraction.
The Cu interconnect layer may be formed in an interconnect trench formed within the first insulating layer, and the metal oxide layer may be formed selectively on the Cu interconnect layer.
The metal oxide layer may be formed by first applying chemical-mechanical polishing to the surface of the Cu interconnect layer, then depositing a metal layer selectively on the Cu interconnect layer by immersion plating, and thereafter heat-treating the metal layer in an oxygen-containing atmosphere.
To address the above challenges, there is provided a method for fabricating a semiconductor device having a multilevel interconnect structure, comprising the steps of: forming a first insulating layer on a semiconductor wafer; forming a Cu interconnect layer on a surface of the first insulating layer; depositing a metal layer on the Cu interconnect layer by immersion plating; forming a metal oxide layer by heat-treating the metal layer in an oxygen-containing atmosphere; and forming a second insulating layer on the metal oxide layer.
The step of oxidizing the metal layer may include the step of heat-treating the semiconductor wafer, including the metal layer, in the oxygen-containing atmosphere for a period not shorter than 30 seconds but not longer than 60 minutes at a temperature not lower than 150° C. but not higher than 450° C.
To address the above challenges, there is provided a method for fabricating a semiconductor device having a multilevel interconnect structure, comprising the steps of: forming a first insulating layer on a semiconductor wafer; forming an interconnect trench in the first insulating layer; embedding a Cu layer into the interconnect trench; cleaning the surface of the Cu layer by chemical-mechanical polishing; depositing a metal layer on the cleaned surface of the Cu layer by immersion plating; forming a metal oxide layer by heat-treating the metal layer in an oxygen-containing atmosphere; and forming a second insulating layer on top of the first insulating layer containing the metal oxide layer.
In the above method, the step of coating an interior surface of the interconnect trench with a film that acts as a barrier layer may be included between the step of forming the interconnect trench and the step of embedding the Cu layer. Further, the step of forming the metal oxide layer may include the step of heat-treating the semiconductor wafer, including the metal layer, in the oxygen-containing atmosphere for a period not shorter than 30 seconds but not longer than 60 minutes at a temperature not lower than 150° C. but not higher than 450° C.
Further, if copper oxide particles are precipitated on the surface of the metal oxide layer in the heat treating step, the copper oxide particles may be removed before proceeding to the step of forming the second insulating film.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.